The present invention relates to a process for producing an insulated rectangular (flat) wire by applying an insulating material onto a round conductor, baking the insulating coating, and rolling the insulated wire into a rectangular or flat shape. The present invention relates particularly to a process for producing an insulated rectangular wire having excellent insulating properties that adapt the wire to edgewise winding.
Rectangular insulated wire manufactured by drawing round insulated wire has been extensively used for voice coils of loudspeakers. The range of application of such wire has recently been expanded to include drive motors for computer-associated equipment. Insulated rectangular wires may be wound into a coil flatwise, but more frequently, they are wound edgewise. Rectangular insulated wires for use in coils must meet the following requirements: (1) Each turn of the coil has a smooth and even surface. This requirement imposes a maximum limit of the amount of variation in the width of the flat insulated wire. (2) The insulating coating is crack free and has no part where the conductor is exposed. This means that there should be no breakage of the insulating coating when pressure is applied in the rolling operation, and that surface flaws in the conductor should not cause cracks in the insulating coating due to mechanical stress occurring during the winding of the insulated wire into a coil. (3) The insulating coating adheres sufficiently strongly to the conductor that no separation of the coating occurs while the round insulated wire is shaped into a rectangular form under a very high pressure.
The present inventors previously filed a patent application for a process for producing a rectangular insulated wire having a very high precision in the dimension of width by applying a coating of an insulating material onto a rigid conductor and baking the insulating coating. However, no satisfactory solutions have been proposed for meeting requirements (2) and (3). That is, if a round conductor has a surface flaw, the insulating coating over that flaw may crack during the process of rolling the round wire into a flat shape. Even a very small flaw in the conductor surface can cause such a defect when the rectangular insulated wire is wound into a coil edgewise. That is, the surface area of each turn of the coil facing the outside is expanded during winding so that the insulating coating tends to crack. On the other hand, the surfaces facing the center of the coil shrink in area as the coil is wound, and hence the possibility of cracking the coating is much smaller than in the coating facing outward. Cracks in the insulating coating unavoidably deteriorate the electrical properties of the coil and impair its reliability and service life by an appreciable degree. Therefore, an improved method for producing a rectangular insulated wire having no cracks in the insulating coating has long been desired.
In order to produce a flat insulated wire having a higher width to thickness ratio, the round wire must be rolled at an increased reduction ratio, but then the strength of adhesion of the insulating coating to the underlying conductor is decreased so as to increase the possibility of separation of the insulating coating from the conductor. If the width to thickness ratio of the wire exceeds about five, the round wire must be subjected to at least two roll passes. Doing so, however, further weakens the adhesion of the insulating coating to the conductor and increases the chance of separation of the insulating coating.
In order to solve these problems, the present inventors have made various studies on a process for producing a rectangular insulated wire by rolling a round wire without causing cracks or separation of the insulating coating from the conductor.